ABB REG670 Applications Manual page 250

Section 8
Impedance protection
244
voltage inversion phenomenon may occur also on remote end of adjacent lines.
Distance protection of such line must have built-in functionality which applies
normally to protection of series compensated lines.
It usually takes a bit of a time before the spark gap flashes, and sometimes the fault
current will be of such a magnitude that there will not be any flashover and the
negative impedance will be sustained. If equation
< < +
X X X X
11 C S 11
EQUATION1898 V1 EN
in figure 124, the fault current will have the same direction as when the capacitor is
bypassed. So, the directional measurement is correct but the impedance measured is
negative and if the characteristic crosses the origin shown in figure
operate. However, if there is a memory circuit designed so it covers the negative
impedance, a three phase fault can be successfully cleared by the distance protection.
As soon as the spark gap has flashed the situation for protection will be as for an
ordinary fault. However, a good protection system should be able to operate correctly
before and after gap flashing occurs.
jX
X
12
X
11
Z
S
X
C
en06000625.vsd
IEC06000625 V1 EN
Figure 124: Cross-polarized
quadrilateral
characteristic
If the distance protection is equipped with an earth-fault measuring unit, the negative
impedance occurs when
× > × +
3 X 2 X X
C 1 _ 11 0 _ 11
EQUATION1919 V1 EN
Cross-polarized distance protection (either with mho or quadrilateral characteristic)
will normally handle earth-faults satisfactory if the negative impedance occurs inside
the characteristic. The operating area for negative impedance depends upon the
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
122
R
RV
R
Z
S
IEC06000584-SMALL V1 EN
Figure 125:
1MRK 502 071-UEN -
is valid
(Equation 122)
124 the IED cannot
jX
X
12
X
11
R
R
FW
X
C
en06000584_small.vsd
Quadrilateral
characteristic with
separate impedance and
directional measurement
(Equation 123)
Application manual